1. Field of the Invention
The present invention relates to a porous metallic sheet to be used as a substrate of a battery electrode; an electrode plate of the battery; and a method of manufacturing the porous metallic sheet and the electrode plate. More particularly, the present invention relates to a porous metallic sheet to be used as the substrate of positive and negative plates of a nickel hydrogen battery, a nickel cadmium battery, a primary lithium battery, a secondary lithium battery, and the like; and an electrode plate formed by applying an active substance to the porous metallic sheet; and a method of manufacturing the electrode plate and the porous metallic sheet.
2. Description of Related Art
Heretofore, as the substrate of the electrode plate comprising positive and negative electrode plates of the nickel hydrogen battery and the nickel cadmium battery, principally, a nickel-plated punched iron metal on which pores have been formed by a press is used. The electrode plate is formed by applying an active substance to the punched metal. A cylindrical battery accommodates the belt-shaped positive and negative electrode plates coiled via a separator, while a rectangular solid battery accommodates the positive and negative electrode plates laminated one on the other via the separator.
As the substrate of the electrode plate comprising the positive and negative electrode plates of the primary lithium battery, a metallic plate (SUS, Ti) formed into a lath net is mainly used. An active substance is applied to the lath net to form the electrode plate. In the case of a secondary lithium battery, the positive plate is formed by applying the active substance in a required thickness to both surfaces of a metallic core made of an aluminum foil, whereas the negative plate is formed by applying the active substance in a required thickness to both surfaces of a metallic core made of a copper foil.
In recent years, a foamed sheet made of resin, a nonwoven cloth made of resin, and a mesh sheet made of resin are chemically plated to allow them to be conductive and then, electrically plated, and then, resin removal and sintering operations are performed to form a porous metallic sheet to use it as the substrate of the electrode plate of the nickel hydrogen battery, the nickel cadmium battery, and the primary lithium battery.
The punched metal used as the substrate of the electrode plate of the nickel hydrogen battery and the like has the following disadvantages:
(1) Portions to be formed into pores are cut off when the punched metal is punched by a press. For example, when the percentage of pores is 50%, half of the material is lost. As such, the production cost is high.
(2) It costs high to operate a press to be used to form pores. (3) Because the pores are two-dimensional, the percentage of pores is 50% at most. Thus, there is a limitation in the amount of an active substance to be applied to the pores.
(4) In order to increase the capacity of a battery, it is preferable to use a thin substrate having a high percentage of the pores so as to apply a large amount of active substance to the pores. But there is a limitation in the percentage of pores for the reason described above. Further, in order to reduce the thickness of the substrate from 60-80 .mu.m to less than 60 .mu.m, material cost is high and the punched metal is plated at a low efficiency. That is, processing cost is high. In addition, if the substrate is thin, it is easily deformed or a burr is easily formed thereon in forming the pores by the press.
In the case of the metallic plate processed into the lath net used as the substrate of the electrode of the primary lithium battery, the metallic plate is deformed and warped due to stress locally concentrated in processing the metallic plate into the lath net. That is, the metallic plate becomes unflattened. The lath net deformed and warped is corrected by a leveler without changing the original size in order to produce the substrate at a low cost. Then, the active substance is applied to the lath net. Then, the lath net is cut to a plurality of pieces having the size which meets the battery standards. At this time, the deformation generated when the metallic plate has been processed into the lath net is regenerated and burrs are likely to occur. As a result, when the lath net is coiled via a separator, there is a possibility that the burr and the deformation cause leakage. It is preferable for the electrode of the primary lithium battery to have a high percentage of pores, so long as the lath net has a preferable strength. But structurally, the lath net should not have the percentage of pores not more than 63%. There is another problem that the higher the percentage of pores is, the higher material cost is.
The porous metallic sheet formed by plating a base porous sheet made of resin, successively burning out and sintering the plated material sheet used as the substrate of the electrode plate developed to replace the punched metal and the lath net has a high percentage of pores and allows a great amount of active substance to be applied to pores thereof. But it is necessary to chemically and electrically plate the porous sheet made of resin and thus perform a complicated process. Hence, the plated porous metallic sheet is produced with an unfavorable productivity. Further, chemicals such as plating liquid is used and high power is consumed. Therefore, producing cost is high. In addition, the management of treatment liquid and countermeasure against pollution are required.
Furthermore, after the surfaces of organic fibers are chemically plated with a conductive substance and the conductive substance is electrically plated in a thickness of 25 .mu.m-50 .mu.m. As a result, the outer diameter of each fiber (mixture fiber) consisting of an organic fiber and plated metal becomes large. When the organic fiber is burnt out continuously the plated metal is sintered, the portion of the fiber occupied with the organic fiber becomes hollow. As a result, a resultant metallic fiber is annular and has a large diameter. That is, the resultant porous metallic sheet has a structure in which pores are surrounded with a framework made of metallic sleeves having cavities.
In order to use the porous metallic sheet as an electrode plate, the active substance is applied to pores of the plated porous metallic sheet. But it cannot be applied to the cavities of the sleeves. Thus, the cavities are reactive portions in the electrode plate. Further, because the outer diameter is each of the sleeves is large, the plated porous metallic sheet has a small volume of the pores surrounded with the metallic cylinders. Therefore, a large amount of the active substance cannot be applied to the plated porous metallic sheet.
When the sheet structure composed of the organic fibers, for example, a nonwoven sheet composed of the organic fibers is electrically plated after it is allowed to be conductive, metal is deposited thickly on the surface of the nonwoven sheet while it is not deposited much in the interior (center portion in thickness direction of sheet), i.e., the deposition amount of the metal in the interior is about half as much as that in the surface. That is, it is difficult to make the amount of a metallic framework on the surface equal to that on the entire sheet.
In order to use the porous metallic sheet as the substrate of the electrode of a battery, as described above, the active substance is applied to pores and thereafter, it is pressurized to adjust the thickness thereof to the standard one. It is easy to apply the active substance to the porous metallic sheet having a large thickness and a high percentage of pores.
According to the conventional art, it is not easy to produce a thick porous metallic sheet formed by coating the surface of the nonwoven sheet made of organic fibers with a conductive substance and electrical plating.
That is, supposing that in the conventional nonwoven sheet comprising the organic fibers, quantity of fiber is 40-50 g/m.sup.2 ; quantity of resin binder is 20 g/m.sup.2 ; the total weight is 60-70 g/m.sup.2 ; and the percentage of pores is 95%, the maximum thickness thereof is 2.5 mm-3.5 mm. After the nonwoven sheet having the above thickness is subjected to electric conduction treatment, electric plating, resin burning out, and sintering, it is difficult to produce a porous metallic sheet having a thickness 1.6 mm.
Further, the conventional nonwoven sheet is produced by using short organic fibers. The short organic fibers are knitted into the nonwoven sheet by a spinning carding machine and resin binder is used to connect the short organic fibers to each other. In the nonwoven sheet composed of the organic fibers thus produced, resin binder (R) is collected at the intersections of organic fibers (f), thus forming lumps at the intersections, as shown in FIG. 28.
When the nonwoven sheet is subjected to electric conduction treatment and electrical plating, the diameters of fibers are large at the intersections. Thus, when the organic fibers are subjected to resin burning out and sintering operations, the resin lumps become hollow at the intersections. Thus, as described above, the active substance cannot be applied to the hollow portions, and thus the hollow portions are unconductive and further, the diameters of the fibers are great at the intersections. Accordingly, the nonwoven sheet has a small volume of pores. Thus., a large amount of active substance cannot be applied to the porous metallic sheet.
The higher the percentage of pores is, the greater the active substance can be applied to the porous metallic sheet to be used as the electrode plate, and thus, the electrode plate is allowed to have a long life. The smaller the area of each pore is, the greater the area of contact between metal and the active substance is. That is, the higher the percentage of pores is and the smaller the area of each pore is, the more favorable the performance of the electrode plate is.
According to the conventional art, in order to reduce the areas of pores in producing the conventional porous metallic sheet by plating the surface of the organic fibers, it is necessary to make the dense meshes of the sheet comprising the organic fibers small. But the surface of each organic fiber is plated and the organic fibers are burnt out. As a result, the resultant porous metallic sheet has a small percentage of pores and many cavities formed as a result of the removal of the organic fibers.
Aluminum foils and copper foils are hitherto used as the substrate of the electrode plate of the secondary lithium battery. The active substance is applied to both surfaces of each solid metallic foil in the same thickness while the metal foil is being drawn. But because the metallic foil has a small strength, the producing line cannot be operated at a high speed. Further, it is not easy to apply the active substance to upper and lower surfaces of the metallic foil in the same thickness. Actually, the thickness of the active substance applied to the upper surface of the foil is often different from that applied to the lower surface thereof. As a result, the active substance is not partly conductive in charge and discharge times and hence the active substance cannot be utilized efficiently in a battery case.
The substrate of the electrode plate which satisfy all the following requirements has not been provided.
(a) The substrate has a high conductivity: The internal resistance of a battery is set to a small value so as to perform electricity-collection action smoothly.
(b) The substrate has a high percentage of pores: A large amount of active substance can be applied to the substrate so as to increase the capacity of the battery.
Even though a large amount of active substance can be applied to the substrate, electricity-collection action cannot be performed smoothly when the area of contact between metal and the active substance is small. Thus, it is necessary for the substrate to have a high percentage of pores and a large area of contact between the metal and the active substance.
(c) The thickness of the substrate is small and the substrate has a great tensile force: If the substrate is thin, the battery can be allowed to have a high performance by accommodating a large amount of substrate in the battery case.
(d) The substrate is provided with a electricity-collection lead and produced in a required configuration at a low cost.